1. Field of the Invention
The present invention relates to floating vessels used for offshore drilling and production of petroleum.
2. Description of the Related Art
Petroleum production often requires the placement of rig in an offshore location. In shallower waters, the rigs and production facilities can be placed on freestanding offshore platforms. As the water becomes deeper, however, these become impractical, and it is necessary to have a floating platform, or support vessel, upon which the rigs and production facilities can be placed.
One type of deepwater support vessel is the tension leg platform (TLP). The TLP is a buoyant platform that is secured to the seabed using generally vertically-oriented rigid tethers or rods that restrain the platform against vertical and horizontal motion relative to the well in the seabed below. Thus, these platforms have a very short period in response to wave action.
An alternative to the TLP is the deep draft caisson vessel (DDCV). The DDCV is a free floating vessel which is moored to the seabed using flexible tethers so that vertical and horizontal motion of the vessel is restrained, although not eliminated. Examples of DDCVs are found in U.S. Pat. No. 4,702,321.
Methods for restraining the DDCVs attempt to slow, rather than eliminate, the natural response period of the vessel to wave effects. Current DDCV arrangements xe2x80x9cdecouplexe2x80x9d the vessel from the individual wells being supported so that the wells are not subject to the same induced motions as the vessel. Decoupling is typically accomplished by using buoyant means to make the wells separately freestanding and using flexible hoses to interconnect the vertical risers from the well to the production facilities.
A common variety of DDCV is the type shown in U.S. Pat. No. 4,702,321 that utilizes a long cylindrical structure and is commonly known as a spar. The long cylindrical shape of the spar provides a very stable structure when the vessel is in its installed position that exhibits very slow pitch surge and heave motions. Heave motion, however, is not totally eliminated, allowing the structure to bob up and down vertically in the sea. Recently, attempts have been made to add a number of horizontally extending plates along the length of the spar in order to help the spar be more resistant to heave.
Regardless of the presence of the plates, the spar must be assembled and transported in a horizontal position and then installed by being upended at or near the final site using a large crane that must also be transported to the installation site. As these caisson structures are often around 650 ft. in length, transport and, particularly upending, of the structure are risky. Further, it is only after a successful upending of the structure has occurred, and the lower portion of the structure has been successfully moored, that components of the rig can be placed atop the spar.
What is needed is a floating vessel which provides reduced motions and slow natural response periods to heave, but also can be assembled and transported in a vertical, or upright, orientation. A vessel of this type would permit rig components and other structures to be placed atop the vessel prior to or during transport.
The present invention provides an improved floating vessel that is capable of being moored by tethers to the sea floor. The vessel has an upper tower section with a reduced diameter or width and an affixed lower base section having an enlarged diameter or width.
The lower section contains ballast distributed upon its lower floor. The lower section also includes flotation tanks which can be filled with air to raise the vessel in the water such that portions of the lower section are raised above the water. Alternatively, the flotation tanks can be flooded to dispose the lower section and a portion of the upper section below the water level.
The upper tower section of the vessel supports a deck structure upon which rig components can be constructed or secured. The tower section includes flotation tanks as well. In preferred embodiments, these tanks are variable tanks that can be partially filled with air and partially flooded with water.
The vessel defines a central chamber within which drilling risers are contained and suspended from the deck structure of the vessel downward toward the sea floor. One or more supports are provided which assist in securing the riser and absorbing energy from movement of the platform.
The floating vessel can be constructed and transported in a upright or vertical orientation so that it does not need to be upended prior to mooring at its intended location. In addition, structures such as rig components may be placed atop the tower portion prior to or during transportation of the vessel. During transportation by towing, the flotation tanks of the lower section are filled with air so that the lower section is partially raised above the surface of the water. The vessel is placed into its installed position by flooding the flotation tanks of the lower section to cause the lower section to become submerged.